In recent years, a power semiconductor module (IGBT, power MOSFET, etc.) capable of high-voltage, large-current operation has been used in the field of an electric vehicle inverter. As a substrate used in the power semiconductor module, a ceramic circuit board obtained by bonding a metal circuit plate to one surface of an insulating ceramic substrate and by bonding a metal radiator plate to the other surface thereof can be used. Further, a semiconductor device, etc., is mounted on the upper surface of the metal circuit plate. Methods for bonding the insulating ceramic substrate, metal circuit plate, and metal radiator plate include an active metal method utilizing a brazing filler metal and so-called a direct bonding copper method by which a copper plate is bonded directly.
In such a power semiconductor module, the heat generation increases when large current is made to flow. However, the insulating ceramic substrate is lower in thermal conductivity than a copper plate, which can become a factor for blocking thermal radiation from the semiconductor device. Further, thermal stress based on a difference in thermal expansion coefficient among the insulating ceramic substrate, metal circuit plate, and metal radiator plate occurs, which may make the insulating ceramic substrate cracked and destroyed, or may be a cause of delamination of the metal circuit plate or the metal radiator plate from the insulating ceramic substrate. As described above, high thermal conductivity and high mechanical strength is required for the insulating ceramic substrate to obtain satisfactory thermal radiation performance. Alumina, Aluminum nitride, and silicon nitride can be cited as a material of the insulating ceramic substrate. Among them, silicon nitride can be used as a material for a ceramic substrate having a higher thermal conductivity and excellent in mechanical strength and is thus suitably used for the power semiconductor module having a structure to which strong stress is applied. The silicon nitride substrate has a plate-like shape having a thickness of about 0.1 to 1 mm. As a manufacturing method of the silicon nitride substrate, a method may be adopted in which bulk silicon nitride ceramics is formed into a substrate by a machining process; however, the silicon nitride ceramics is difficult to process, resulting in high cost. Thus, more suitably, a sheet-like formed body is previously produced, followed by sintering to obtain a silicon nitride substrate. As a method for manufacturing the sheet-like formed body, press molding, extrusion molding, and doctor blade molding can be cited. Among them, the doctor blade molding is preferably used because of high mass productivity. However, the doctor blade molding is a molding method in which slurry composed of a ceramic raw material powder, solvent, binder, and the like is made to pass through a slit formed by blades, followed by drying, so that a defect such as crack or wrinkle is easily generated by stress caused due to shrinkage at the drying stage. In order to suppress occurrence of such a defect, the slurry is made to contain a large number of binders for maintaining the binding between raw material powder particles. The larger the specific surface area of the raw material powder used is, the larger number of binders is required for maintaining the binding between the raw material powder particles. However, the binder needs to be burned away in a degreasing process before sintering. Thus, when the amount of the binders to be contained is large, the degreasing may become difficult. The contained amount of the binder is limited for the above reason, so that, in the case where a raw material powder having a comparatively large specific surface area is used, the binder amount does not suffice to cause a defect in the formed body during a drying process at the molding time, which may easily cause a molding crack. In general, in the case of the silicon nitride substrate, a sheet-like formed body having a larger area than the area of a circuit board used as a product is produced, followed by degreasing and sintering, and is finally divided for use, so that it is necessary to extract a product portion from a crack-free area where a molding crack or degreasing crack has not occurred (although the area where the crack has occurred can be subjected to burning). This degrades the production yield, resulting in production cost increase.
Thus, a raw material powder having a comparatively small specific surface area is suitable for manufacturing a defect-free formed body by using the doctor blade method. However, due to comparatively small specific surface area of the raw material powder, sintering performance becomes degraded when the silicon nitride powder hard to be sintered is used, and thus a high density and high thermal conductivity silicon nitride ceramic substrate cannot be obtained.
In order to improve the sintering performance of the silicon nitride ceramics, Patent Document 1 discloses a crystalline silicon nitride powder having a silicon oxide layer as the top layer and a silicon oxynitride layer under the silicon oxide layer, the amount of oxygen existing in the silicon oxide layer being ≦0.1 wt % (expressed in terms of oxygen content) and the amount of oxygen existing in the silicon oxynitride layer being 0.4 to 1.2 wt % (expressed in terms of oxygen content) and having a specific surface area of 5 to 30 m2/g. By using this powder, a silicon nitride formed body easy to sinter and excellent in characteristics of a sintered body such as strength at high temperature can be obtained.
Further, the present inventor has disclosed a silicon nitride powder for developing high mechanical strength performance and high conductive performance and its manufacturing method in Patent Document 2. By using a silicon nitride-based powder having a β fraction of 30 to 100%, an oxygen amount of 0.5 wt % or less, an average particle diameter of 0.2 to 10 μm, and an aspect ratio of 10 or less, it is possible to obtain a high-thermal conduction silicon nitride-based sintered body having excellent mechanical strength and enhanced thermal conductivity more than before without the anisotropy in the direction of thermal conduction.